Fix BT crash due to unset value for BluetoothGattDescriptor from API user

Validate writeDescriptor and writeCharacteristic methods at API invocation
for non null initialisation.

Bug 18395071

Change-Id: I411a57b77981310d8db1f98c67e03b4327c93339
1 file changed
tree: 509c10730f321558a357f7c3ad74e3e89dff4638
  1. framework/
  2. service/